1. Field of the Invention
The present invention relates to telecommunications, and, in particular, to wireless communications systems conforming to a code-division, multiple-access (CDMA) standard, such as the cdma2000 standard of the IS-95 family of CDMA wireless standards.
2. Cross-Reference to Related Aapplications
This application is one of a set of U.S. patent applications consisting of Ser. Nos. 09/330,888, 09/330,582, 09/332,274 and Ser. No. 09/330,506 all of which were filed on the same date and the teachings of all of which are incorporated herein by reference.
3. Description of the Related Art
FIG. 1 shows a block diagram of a conventional CDMA wireless communications system 100. Communications system 100 is assumed to conform to the cdma2000 standard in the IS-95 family of CDMA wireless standards, although the present invention is not necessarily so limited. Communications system 100. comprises an interworking function (IWF) 102 connected to a radio link protocol (RLP) function 104, which is in turn connected to a frame selection/distribution (FSD) function 106, which is in turn connected to one or more base stations 110 via back haul facilities 108 (e.g., T1 lines). Depending on the specific implementation, IWF function 102, RLP function 104, and FSD function 106 may be, but need not be, physically separate functions.
Each base station 110 is capable of simultaneously supporting wireless communications with one or more mobile units 112. FSD function 106 performs a forward-link frame distribution function in which frames of data corresponding to user messages are distributed to the various base stations. In addition, FSD function 106 performs a reverse-link frame selection function in which frames of data received from the various base stations are processed for forwarding on to RLP function 104. In the forward-link direction, RLP function 104 segments user messages received from IWF function 102 into frames of data for distribution by FSD function 106. In the reverse-link direction, RLP function 104 reassembles packets of data received from FSD function 106 into user messages for forwarding on to IWF function 102. IWF function 102 implements a high-level point-to-point protocol (PPP) to perform certain centralized functions for communications system 100 to coordinate and control operations at the various base stations 110. IWF function 102 also functions as the interface between communications system 100 and other communications systems (not shown) to provide a full range of telecommunications services to the mobile units, including voice communications with a remote end unit and/or data communications with a computer server or other nodes of a computer network.
As used in this specification, the term xe2x80x9cmobile unitxe2x80x9d as well as its synonyms xe2x80x9cmobile user,xe2x80x9d xe2x80x9cmobile,xe2x80x9d and xe2x80x9cuser,xe2x80x9d will all be understood to refer to any end node communicating via wireless transmissions with one or more base stations of a wireless communications system, whether that end node is actually mobile or stationary. Also, as used in this specification, the term xe2x80x9cbase stationxe2x80x9d is synonymous with the terms xe2x80x9ccall legxe2x80x9d (or xe2x80x9clegxe2x80x9d for short) and xe2x80x9ccell sitexe2x80x9d (or xe2x80x9ccellxe2x80x9d for short).
The cdma2000 standard supports different modes of data communications. For relatively low rates of data messaging, a fundamental channel (FCH) can handle both signaling and data messaging. Signaling refers to the communications between a mobile and a base station that are used by the mobile and the base station to control the communications links between them, while messaging refers to the information passed through the base station to and from the end nodes of those communications, where the mobile is one of those end nodes. For high-rate data messaging, a supplemental channel (SCH) can be used for data messaging, while the fundamental channel handles the signaling between the mobile and the base station. Alternatively, when an SCH is used for data messaging, the signaling between the mobile and the base station can be handled by a special communications channel called a dedicated control channel (DCCH), which requires less power to transmit than an FCH, which is designed to handle low-rate data messaging in addition to signaling.
FIG. 2 shows a functional block diagram of a portion of communications system 100 of FIG. 1 for a mobile unit 112 operating in soft handoff with three base stations 110. Soft handoff refers to a situation in which a mobile unit is simultaneously communicating with two or more base stations, each of which is referred to as a call leg of those communications. Frame selection/distribution function 106 supports the soft handoff communications between mobile unit 112 and the three base stations 110.
During normal voice communications, mobile 112 transmits voice messages using a reverse-link fundamental channel. Each of the three base stations 110 in soft handoff with mobile 112 receives the reverse-link FCH, accumulates voice messages into reverse-link packets, and transmits the reverse-link packets over back haul 108 to FSD function 106. FSD function 106 receives the reverse-link packets from all three base stations, identifies sets of corresponding reverse-link packets (one reverse-link packet from each base station corresponding to the same voice messages received from the mobile), and selects one reverse-link packet from each set of corresponding reverse-link packets to transmit to the rest of the wireless system for eventual transmission to the remote end of the call (e.g., a connection with a regular PSTN user or possibly another mobile unit in communications system 100).
At the same time, FSD function 106 receives forward-link packets containing voice messages from the remote end of the call intended for mobile unit 112. FSD function 106 distributes copies of each forward-link packet to all of the base stations currently in soft handoff with the mobile. Each base station transmits the forward-link packets to mobile unit 112 using a different forward-link fundamental channel. Mobile unit 112 receives all three forward-link FCHs and combines corresponding voice messages from all three forward-link FCHs to generate the audio for the person using mobile unit 112.
The timing of the distribution of the copies of the forward-link packets from FSD function 106 to the three base stations is critical, because mobile unit 112 needs to receive each set of corresponding voice messages from all three forward-link signals within a relatively short period of time in order to be able to combine all of the corresponding voice messages together. Similarly, FSD function 106 needs to receive all of the corresponding reverse-link packets from the different base stations within a relatively short period of time in order to coordinate the selection of packets for further processing.
In order to satisfy these forward-link and reverse-link timing requirements, whenever a new call leg is added at a base station (i.e., whenever a new base station begins communications with a particular mobile unit in soft handoff), special synchronization procedures are performed between the base station and FSD function 106, e.g., in order to ensure proper synchronization of that base station""s forward-link transmissions with the forward-link transmissions from the other base stations currently participating in soft handoff with the mobile. These synchronization procedures involve specific communications back and forth between the base station and the FSD function over the back haul.
Although a fundamental channel can support some modest amount of data messaging in addition to voice messaging, the cdma2000 standard also supports high-speed data messaging via supplemental channels. According to the cdma2000 standard, since data messaging is typically bursty (i.e., intermittent), as opposed to the continuousness of voice messaging, supplemental channels are established and maintained only for the duration of each data burst. During a burst of data messaging via an assigned SCH, the mobile unit is said to be in an active state. Between bursts of data messaging when no SCH is currently assigned, but when an FCH (or DCCH) is assigned, the mobile unit is said to be in a control hold state. When no dedicated air interface channels are assigned, the mobile unit is said to be in a suspended state.
Analogous to the use of a fundamental channel for voice and/or low-speed data messaging, high-speed reverse-link data messages are transmitted by mobile unit 112 using a reverse-link supplemental channel. Each base station currently operating in soft handoff with the mobile unit receives the reverse-link SCH and generates reverse-link packets of data messages for transmission to FSD function 106 via the back haul. FSD function 106 receives the reverse-link packets from all of the base stations and selects appropriate reverse-link packets for transmission to the remote end of the call (which, in the case of data messaging, may be a computer server).
Similarly, FSD function 106 receives forward-link packets of data messages intended for mobile unit 112 and coordinates the distribution of those forward-link packets via the back haul to the appropriate base stations for coordinated transmission to the mobile via assigned forward-link supplemental channels. In addition to the synchronization processing between each base station and FSD function 106 required to meet the timing requirements for receiving messages at the mobile, in data communications, the base stations need to coordinate their operations to ensure that they all transmit their forward-link SCHs to the mobile at the same data rate. This requires the base stations to communicate with one another via the back haul whenever a new burst of forward-link data is to be transmitted to the mobile unit requiring new SCHs to be assigned.
The reactivation time is the time that it takes to change the status of a mobile unit from either the suspended state or the control hold state to the active state in which a high-data-rate air interface channel is assigned. In the suspended state, no dedicated air interface channel is assigned to the mobile unit. In the control hold state, the mobile unit is assigned only a dedicated power control and signaling channel. In prior-art IS-95 CDMA systems, the reactivation time includes the time required to assign a new channel to the mobile and the time required to synchronize each base station with the frame selection/distribution function. When the new channel is a supplemental channel to be used for data transmission to a mobile unit in soft handoff, the reactivation time also includes the time required for the different base stations to coordinate their forward-link transmission data rates. In general, the longer the reactivation time, the lower the data throughput of the wireless system. As such, it is desired to keep reactivation time as low as practicable.
The back-end architecture, also referred to as the back haul, for prior-art IS-95 CDMA wireless systems is based on providing voice service in a wireless environment that supports soft handoff (SHO) on both forward and reverse links. Voice service is implemented using a vocoding function that is provided, for example, in the centralized location of the mobile switching center (MSC), and these resources need to be assigned and freed as calls are set up and cleared. The prior-art voice-oriented back haul is also used to provide circuit-switched data service and has also been applied to packet data service. The rationale for using the existing voice-oriented back haul for packet data service is to save on development cost and time, because much of the existing structure and operation can be reused. The penalty, though, is to force larger-than-necessary delays on the packet service because of the many set up, clearing, and synchronization operations that are carried through to the packet service, which result in large reactivation times during packet data service.
Problems with Using Existing Back Haul Architectures for Packet Data Service
The following problems occur when the existing circuit-oriented techniques for back haul transport are used to support packet data, rather than the voice and circuit-mode data applications they are designed to handle.
1. When a mobile call is initially set up, a frame selection/distribution function is chosen by the wireless system software to service the call, and an initialization and synchronization procedure occurs between the FSD function and the base station serving the call. The synchronization procedure involves exchanging null (no information) packets between the FSD function and the (primary) cell for a number of 20-millisecond intervals, until synchronization is achieved. Timing adjustment messages may need to be exchanged between the primary cell and the FSD function before synchronization can be achieved.
These procedures add unnecessary delay when applied to a packet data call. Packet data calls are generally more tolerant to transmission delays than are voice or circuit-mode data calls. If the circuit-oriented itialization procedure is applied to a packet data call, an extra delay is added to the time it would otherwise take to bring the user from a suspended state, in which no air interface channels are assigned to the user, to an active state, in which at least one air interface channel is assigned, and the mobile user can begin sending user messages to the FSD function.
2. When secondary legs are added to a call, interactions between the secondary cells and the FSD function need to occur before user messages can be transferred from a secondary leg to the FSD function. Hence, these circuit-oriented procedures on the back haul add delay when legs are added to a call.
3. FSD function transmissions to the cell are synchronized to the 20-millisecond boundaries of the air interface transmissions. This arrangement, among other things, avoids contention and delay at the cells, and saves on the memory that would otherwise be needed to buffer user messages before their transmission over the air interface. User messages arrive at the cell at just about the time they need to be transmitted over the air interface. Such synchronization is required for voice calls, but might not be required for data calls, unless the forward link of the data call has multiple call legs, in which case, synchronization is required, since all legs must transmit a given user message over the air interface at precisely the same time instant. Also, like all circuit-oriented procedures, when used to transport packet data having bursty arrival statistics, back haul bandwidth is wasted.
4. The radio link protocol as currently defined in standards (e.g., Interim Standard IS-707) performs the function of ensuring reliable exchange of user messages between the network and the mobile unit. It has provisions to retransmit data received in error, or data missed by the receiver, and also to discard duplicate received messages. Prior art for this protocol is to have the network-based end of the RLP function coordinate its transmission of information to the base station with the rate and format used to transmit user messages over the air. For circuit-mode data, this arrangement works well, because the rate and format are determined when the call is established, and do not change during the call. However, for a high data rate packet mode data service, the scarce air interface resource is assigned only when there is data to exchange with the mobile user. The air interface channels are allocated and de-allocated as needed by the various packet data users. Hence, prior art demands that the network-based RLP function coordinate its transmission of data with the base stations prior to sending data to the base stations. This coordination means that delay is added between the time user data arrives at the RLP function and the time the data is sent to the base stations for transmission over the air to the user. Furthermore, if a packet data user is inactive for a relatively long period of time (a parameter fixed by each vendor, but could be on the order of 30 seconds), prior art would have the RLP functionality disconnect from the mobile user. Hence, when data again needs to be exchanged with the mobile user, an additional time delay is incurred to re-initialize the mobile unit with the RLP function.
These enumerated problems point out that applying the circuit mode back haul procedures of the prior art to a high-speed packet data (HSPD) service causes substantial delays to the high-speed packet data service. It is therefore desirable to design a back haul architecture that (a) is optimized for packet data service and (b) minimizes the reactivation time of users due to back haul procedures.
Power Control
According to the cdma2000 standard, each base station 110 monitors the receive power level of the reverse-link channel signals transmitted by mobile unit 112. Each different forward-link FCH (or forward-link DCCH) transmitted from each base station to the mobile contains a periodically repeated power control (PC) bit that indicates whether that base station believes the mobile should increase or decrease the transmit power level of its reverse-link channel signals. If the current PC bits in a forward-link FCH indicate that the mobile should decrease its transmit power level, the mobile will decrease its transmit power level, even if the current PC bits in all of the other forward-link FCHs from the other legs of the soft handoff indicate that the mobile should increase its power level. Only when the current PC bits in the forward-link FCHs from all of the legs indicate that the mobile should increase its transmit power level will the mobile do so. This power control technique enables the mobile to transmit at a minimal acceptable power level in order to maintain communications while efficiently using the possibly limited power available at the mobile and reducing the possibility of interference at the base stations with reverse-link signals transmitted from other mobile units.
FIG. 3 shows a mobile unit 302 in soft handoff with two base stations 304 during conventional reverse-link data transmissions from the mobile unit. According to the prior-art IS-95 standards, a symmetric active set must be maintained by the forward and reverse links. In other words, the set of base stations currently participating in soft handoff with a particular mobile unit in the forward-link direction must be identical to the set of base stations currently participating in soft handoff with that same mobile unit in the reverse-link direction.
The soft handoff situation shown in FIG. 3 satisfies this requirement. In particular, in the forward link, each base station 304 simultaneously transmits in the forward-link direction using either a forward dedicated control channel (F-DCCH) or a forward fundamental channel (F-FCH). At the same time, mobile unit 302 transmits in the reverse-link direction using a reverse DCCH, a reverse FCH, and/or a reverse supplemental channel, and those reverse-link signals are simultaneously received and processed in parallel at both base stations. Thus, the active set for the forward link (i.e., base stations A and B) is identical to the active set for the reverse link. During the active state, each base station generates power control bits constituting a power control sub-channel that is multiplexed (i.e., punctured) either on the corresponding F-DCCH or on the corresponding F-FCH, depending on which channel is present.
The present invention is directed to a technique for transmitting power control signals from a base station to a mobile, where the mobile uses those power control signals to control the transmit power level of its reverse-link channels. For cdma2000 systems, the PC bits of the power control sub-channel are transmitted using the common power control channel, which is decoupled from any other forward-link signals transmitted from that base station to that mobile. The ability of base stations to use decoupled forward-link channels in order to transmit their power control signals to a mobile enables a mobile to operate with different active sets for the forward and reverse links. This enables forward-link data traffic to be implemented using a simplex mode, even when the mobile is operating in soft handoff in the reverse link. This in turn greatly reduces the reactivation time involved in transitioning a mobile from either the suspended state or the control hold state to the active state, which is particularly desirable for bursty (i.e., intermittent) packet data flow, as opposed to continuous circuit-oriented voice messaging.
In one embodiment, the present invention is a wireless communications method, comprising the steps of (a) beginning to receive, at a first base station of a wireless communications system, one or more reverse-link channels; and (b) then transmitting corresponding forward-link power control signals corresponding to a power control sub-channel, by the first base station, using a decoupled forward-link channel that is decoupled from any other forward-link transmission by the first base station.
In another embodiment, the present invention is a wireless communications system comprising a first base station configured to (a) begin to receive and process one or more reverse-link channels; and (b) then transmit corresponding forward-link power control signals corresponding to a power control sub-channel using a decoupled forward-link channel that is decoupled from any other forward-link transmission transmitted by the first base station.